Optical image scanners for scanning an image to be stored or reproduced are well-known in the prior art. Such optical scanners are typically used in laser printers for scanning a laser generated light beam across a receiving or recording medium with image information effected by modulating the laser source of the light beam with an image signal and scanning the recording medium with the modulated light beam to form a visible image thereon, either directly or through subsequent processing of the medium. Similar scanners are used in optical character recognition and the like to read an object, image or text for storage or reproduction.
In the known image scanning reading/writing systems, the light beam from the laser source is deflected by a rotating mirror, typically a high speed rotatable polygon having a number of facets, which deflects the incident light beam in a flat tangential field across the scanned image or the receiving medium. Optical lens systems are typically employed in the optical path of the incident light beam to shape it so as to achieve the desired beam profile at the image or receiving medium. Often an additional lens system is employed, in the optical path of the reflected beam, to correct any beam position errors in the cross-scan direction resulting from angular misalignment of the facets of the polygon shaped mirror and to provide a relatively straight, distortion free line scan. Typically in such optical scanners, the laser light beam source and its associated beam shaping lens system are fixed to direct the light beam in an optical path intersecting the "waist" of the rotating mirror facets. Therefore as the mirror rotates, the angle of incidence of the source light beam to the mirror surface changes, resulting in the angular deflection or scanning of the reflected light beam in the desired flat tangential field.
Typical optical scanners are disclosed in commonly assigned U.S. Pat. Nos. 4,796,962 and 4,921,320, incorporated herein by reference in their entireties, and the scanners described in the prior art patents referenced therein. The optical scanners disclosed in the '962 and '320 patents are employed for recording images on receiving mediums which may constitute intermediate image retaining mediums, e.g., xerographic drums, or hard copy prints of pictorial or alphanumeric information.
The '320 and '962 patents are directed to improvements in the optical system for minimizing spot size variations and achieving relatively straight line scans and flat tangential fields for creating half-tone images of good quality.
The polygon-shaped, rotating mirror employed in the prior art optical scanners is typically manufactured of a highly polished metal, e.g., aluminum, or a metallic coating of silver or aluminum on glass which are either left uncoated or else are overcoated with a protective dielectric coating of a metal oxide or fluoride deposited on the surface to protect the mirror surface from damage and oxidation. The thickness of this coating, if specified at all in relation to reflectance, is typically chosen to provide enhanced reflectance at some particular wave length of incident light radiation and angle of incidence. The reflectance of the mirror, be it a metallic mirror or a metallic coating on glass, is a function of the angle of incidence of the light beam. In a typical optical scanner/printer, this angle of incidence may vary over a large range depending upon the focal length of the optical system and the format size of the scanned image or receiving medium, and the resulting reflectance may likewise vary by several percent over the optical scan. The resulting reflectance variation with scan angle introduces a non-uniformity in printed or scanned images, as the intensity of the reflected light beam varies. If the image uniformity is not acceptable, correction may require incorporation of elaborate calibration and compensation technologies.
This problem with reflectance variation as a function of scan angle is noted in U.S. Pat. No. 4,930,869 which provides a light-transmissive plate angularly oriented across the incident and reflective light beam optical paths having an anti-reflection coating thereon. Employing S-polarized or P-polarized incident radiation, the transmittance of the glass plate and the angle of inclination of the glass plate are selected to compensate for the change in reflectance of the mirror facets as the incident angle changes during rotation. Such an arrangement partially compensates for the deflector mirror's reflectance variations with angle of incidence but requires additional structure enclosing the polygon shaped reflecting mirror. The reflecting coating reflects a portion of the incident light beam as stray light that must be baffled from the optical path of the scanned light beam.
In a further embodiment described in the '869 patent, the glass plate is replaced by a further reflecting mirror which itself reflects the scanning light beam reflected by the rotating mirror facets. These embodiments of the '869 patent themselves introduce both complexity and potential aberrations in the desirable flat tangential field, straight line scan and distortion described above. Moreover, they increase stray light reflectance, ghost images, and intensity losses at the interfaces created by the glass plate or the additional mirror.
The polygon-shaped mirrors presently being utilized in many printer-scanner systems consist of an aluminum substrate, carefully machined and highly polished to present optically flat facets, with a dielectric coating deposited on the facets to protect the mirror surface from oxidation. Often, the aluminum substrate is coated with a one-half wave "optical" thickness of SiO.sub.2. The purpose of this silicone dioxide coating is to prevent degradation of the mirror's reflectivity by oxidation, to enhance the reflectivity of the mirror, and to provide a hard coating which protects the surface finish from degradation due to the environment and cleaning operations. The thickness of the dielectric coating, if specified at all, is generally chosen to be equal to L.sub.0 /2N, where L.sub.O is the wave length of the incident radiation and free space and N is the index of refraction of the homogeneous protective coating layer, as taught, for example, in U.S. Pat. No. 4,662,722.
This recommended coating thickness is known to i obtain the highest average reflectance at normal incidence and is referred to as the HWOT (1/2 wave length optical thickness). This particular thickness also happens to provide minimum relative phase shift between the reflected S- and P-polarization components. The '722 patent is directed to specifying such a thickness for minimizing the relative phase shift between the reflected S- and P-polarization components. Such a thickness specification thus provides high reflectance at the normal angle of incidence but does not address the fall off in reflectance as the angle of incidence varies between the minimum and maximum angles encountered in the angular displacement of the rotating mirror facet during a single scan of the incident light beam.
A further U.S. Pat. No. 4,322,130 sets forth a number of coated mirror embodiments of various metal substrates and metal oxide and fluoride coatings where the dielectric coatings are specified as having thicknesses optimized to achieve a desired phase difference between the reflected S- and P- polarization components of an incident polarized light beam. The coated mirrors are employed to reflect light incident at 45.degree. to normal and the angle of incidence does not change.